Parasitic oscillation elimination



Aug. 5, 1941. u M. LEEDS 2,251,629

PARASITIC OSCILLATION ELIMINATION Filed July 16. 1940 I I LI L INDUCTIVEInventor: Laurence M. Leeds,

His Attorney.

Patented Aug. 5, 1941 PARAsrrIc OSCILLATION ELIMINATION Laurance M.Leeds, Rotterdam Junction, N. Y.,

assignor to General Electric Company, a. corporation of New Yorkelimination July 16, 1940, Serial N6. 345,768

(c1. ris -171) Claims.

My invention relates to high frequency circuits and more particularly tomeans for damping undesired oscillations in such high frequencycircuits.

In circuits carrying high frequency current it frequently arises thatcertain portions of the circuit must be made of such length as to haveappreciable inductance, such as .to resonate, with circuit capacitiesand produce undesired oscillations. It has been proposed to shunt such ahigh frequency circuit by a resistance to damp out the undesiredoscillation. In certain cases, however, the resistance itself placed inshunt to the conductors, may have an amount of inductance comparable tothe inductance of the conductor itself. It is accordingly an object ofmy invention to provide an improved and simplified high frequencycircuit in which undesired oscillations are minimized.

, It is a further object of my invention to provide a high frequencycircuit having improved and simplified means for damping undesiredoscillations therein.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,both as to its organization and method of'operation, together withfurther objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing in which Fig. 1

of the drawing illustrates a preferred embodiment In Fig. 1, a pair ofelectron discharge devices lb and H are connected to operate in balancedor push-pull relation to amplify a wave from a source, not shown, whichwave appears across an input inductance l2. The amplified wave isimpressed across an output inductance I3 and is eventually utilized in aload circuit, not shown.

In the input circuit for the devices I0 and .H the ends of theinductance I2 are connected respectively to the control electrodes [4and of the devices 10 and II. The cathodes It and [1 are connected toground.

The central point of the inductance I2 is connected through a choke coilI8 and a source [9 terelectrode and circuit capacities, resonate withthe inductance 12 at the frequency of the wave impressed thereon.

In the output circuit of the devices l0 and I 1, the anodes 23 and 24thereof are connected to opposite ends of the inductance 13,respectively through conductors 38 and 39; A condenser 25 is connectedfromthe anode 23 to ground and a second condenser 26 is connected fromthe anode 24 to ground. These two condensers in series, together withunavoidable interelectrode and circuitcapacities, resonate with theinductance 13 at the frequency of the wave impressed on the inductancel2; The central point of the inductance 131's connected through a chokecoil 21 and a source 28 of anode potential, by-passed by a condenser29,to ground.

In high power amplifiers, in which my present invention is primarilyuseful, triode electron discharge devices are commonly used whichrequire neutralization. Conventional neutralizing means including a pairof condensers 38 and 3| are provided to neutralize the devices I 0 and II. The

condenser 30 is connectedfrom the anode 23 througha conductor 32 to thecontrol electrode l5 of the discharge device I I. Similarly thecondenser 3| is connected from the anode 24 through a conductor 33to thecontrol electrode M of the device It). I I

In the operation of such an amplifier at high frequencies it has beenfound that the conductors 32 and 33,; which often must be ofconsiderable length, may have an inductance resonant with theinterelectrode capacities in the devices ID and Hand withinductance andcapacity in other portions of the circuit at a frequency or frequencies,near or in the band of frequencies within which it is desired to operatethe amplifier. Parasitic oscillation may, of course, be produced at anyfrequency determined by circuit conditions. Such undesired or parasiticoscillations may completely destroy the usefulness of the amplifier ifthe oscillations are strong, or may result in spurious radiations andreduced useful output if the oscillations are weak. For satisfactoryoperation, parasitic oscillations must be completely eliminated. It hasbeen proposed to insert damping resistances in the conductors 32 and 33,but such resistances absorb substantial amounts ofpower of the frequencywhich it is desired to amplify. It has also been proposed to connect aresistance in shunt to some portion of ,each; circuit in which undesiredoscillations persist, sothat the circuit is critically damped andundesired oscillations cannot build up. It has been found, however, thatthe shunt resistance 34 and the condenser 35 may therefore be said to beseries resonant. Likewise the resistance 36 and the condenser 3'! may becalled series resonant. The resistors 34 and 36, being in seriesresonant circuits, therefore act as pure resistances and effectivelydamp oscillations which tend to occur through the conductors 32 and 33.

In the usual case encountered in practice, parasitic oscillationsoccurring in the neutralizing circuit are at frequencies substantiallyhigher than the operating frequency. With resistors 34 and 36 and theirassociated condensers 35 and 31 series resonant at the parasiticfrequency and connected in shunt to conductors 32 and 33, there issubstantially no damping at the operating or power frequency, yet thecircuit may be critically damped at the frequency of undesiredoscillations.

If desired, such a series resonant combination of resistance andcapacity may be connected in shunt to some other portion of a circuitresonant at an undesired frequency, such other portion being, forexample, an anode conductor 38 or 39, or a portion of the choke coil I8or 21, or a surface or portion of a neutralizing condenser 33 or 3|.

In Fig. 2 there is illustrated apparatus similar to that of Fig. l inwhich such a series combination of resistance and capacity is connectedin shunt to a portion of the anode circuit. In this figure many of theelements are like those illustrated in Fig. 1 and are given likereference characters. The devices Hi and II, however, respectively havescreen electrodes 40 and 4!, in addition to anodes 23 and 24, cathodesl6 and I7, and control electrodes I4 and 15. These screen electrodesminimize capacitive reaction between the anodes 23 and 24 and thecontrol electrodes hi and I5. gether and through a source 42 ofpotential to ground and are respectively by-passed to ground throughrespective condensers 43 and 44. Such screen electrodes 43 and 4| arefrequently utilized to avoid the necessity of neutralization.

Very high frequency parasitic oscillations are frequently observed insuch an electron ,diS- charge amplifier. Such oscillations may occur ina path through the conductor 33, the space between anode 23 andelectrode 40, condenser 43, and thus through ground and condenser 25back to the conductor 38. A similar circuit exists through the conductor39, the space between the anode 24 and the electrode 4!, the condenser44 to ground, and thus through the condenser 26 back to the conductor39. Such circuits usually tend to oscillate at a very high frequency.The inductance of the conductors 38 and 39 plays a large part in theseoscillating circuits.

In order to eliminate such undesired parasitic oscillations, a seriescombination of a capacitor 45 and a resistor 43 is connected in shunt tothe conductor 38, and a similar series combination of a capacitor 41 anda resistor 48 is connected in shunt to the conductor 39. The capacitor45 They are connected tois adjusted to resonate at the undesiredparasitic frequency with the inductance of the resistor 46, which existsdue to its finite length. The series combination of capacity andresistance, being series resonant, presents a pure resistance at theundesired frequency in the parasitic oscillation circuit. Thisresistance 4'6 may be adjusted to produce critical damping of theundesired parasitic oscillation, so that such oscillation is effectivelyeliminated. The series combination of capacitor 41 and resistor 43 issimilarly made series resonant, so that it likewise presents a pureresistance at the undesired parasitic frequency in shunt to theconductor 39. The resistance 43 also may be adjusted to produce criticaldamping in the circuit for the parasitic oscillation.

It may be noted that conventional neutralizing means 30, 3i are shown inaddition to the screen electrodes 4!) and 4!. When such screenelectrodes are used at very high frequencies, it is difficult orimpossible to ground them for the high frequency currents andconsequently some feedback is experienced to the control electrodes I4,15. The neutralizing condensers 33, 3| are provided to improve theoperation at such high frequencies.

While I have shown a particular embodiment of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto, sincedifferent modifications may be made both in the circuit arrangement andinstrumentalities employed, and I aim by the appended claims to coverany such modifications as fall within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. Radio apparatus comprising a circuit in which oscillations of anundesired frequency tend to exist, said circuit including a conductor offinite length, which thereby exhibits inductance, across which suchoscillations appear, and a resistance and a capacitor serially connectedbetween spaced points on said conductor, said resistance, due to itsfinite length, being inductive and being resonant at said undesiredfrequency with said capacitor, whereby a pure resistance is presentedbetween said spaced points at said undesired frequency effectively todamp said oscillation.

2. In a high frequency power amplifier, an electron discharge devicehaving an anode, a cathode, and a control electrode, a circuit connectedbetween said anode and control electrode and including a conductor ofsubstantial length, said circuit having a tendency to break intooscillation at an undesired frequency, said conductor due to its lengthhaving substantial inductance, and a series combination of resistanceand capacity connected in shunt to said conductor, said resistance beinginductive due to its finite length and being resonant at said undesiredfrequency with said capacity effectively to damp the oscillation in saidcircuit.

3. An electron discharge amplifier including a circuit which tends tooscillate at an undesired frequency, a portion of said circuit being aconductor of finite length whereby it is inductive, and a seriescombination of resistance and capacity connected in shunt to saidconductor, said resistance also being inductive due to its finite lengthand being series resonant at said undesired frequency with said capacityeffectively to damp said oscillation.

4. In a high frequency power amplifier having a triode electrondischarge device and neutralizing means therefor, said neutralizingmeans including a conductor of substantial length, the inductance ofwhich tends to resonate with other portions of said amplifier at anundesired frequency, and means in shunt to said conductor for dampingoscillations of said undesired frequency, said means comprising a seriescombination of resistance and capacity, said resistance being of finitelength and exhibiting inductance, and said capacity being of such valuewith respect to said inductance as to resonate therewith at saidundesired frequency effectively to damp said neutralizing means.

5. In combination, an electron discharge amplifier device having ananode and a pair of control electrodes, said anode and one of saidcontrol electrodes being adjacent each other, a circuit through saidanode and said adjacent control electrode including a conductor offinite length which thereby exhibits inductance, said circuit tending tooscillate at an undesired frequency, and a series combination of aresistor and a condenser in shunt to said conductor, said resistorhaving finite length and exhibiting inductance and said condenser beingresonant therewith at said undesired frequency effectively to damp saidcircuit.

' LAURANCE M. LEEDS.

